This invention is related generally to electrodes for monitoring tissue electrical activity.
Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various methods have been used in attempting to determine epileptogenic foci, and all, of course, involve sensing of cortical electrical activity using electrical contacts applied in various ways.
Standard scalp contacts have been used for many years, but accurate localization is usually very difficult with recordings obtained from such contacts. Therefore, many epilepsy centers in recent years have used intracranial recording techniques to better define regions of cortical epileptogenicity.
Intracranial recording techniques have used either of two different types of electrodes--intracortical depth electrodes or subdural strip electrodes. The far more commonly used technique of intracranial recording uses intracortical depth electrodes, but other techniques using subdural strip electrodes, first utilized many years ago, have been shown to be relatively safe and valuable alternatives.
The relative safety of subdural strip electrodes lies in the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures with contact bands spaced along their lengths. Such electrodes are inserted into the brain in order to establish good electrical contact with different portions of the brain. Subdural strip electrodes, on the other hand, are generally flat strips supporting contacts spaced along their lengths. Such strip electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain.
A typical subdural strip electrode of the prior art is shown in FIGS. 1-3 and is disclosed in U.S. Pat. No. 4,735,208. The '208 patent discloses a subdural strip electrode 10 having an elongated flexible silicone dielectric strip 14, a plurality of spaced aligned flat electrical stainless steel contact disks 16 held within dielectric strip 14, and lead wires 18 exiting strip 14 from a proximal end 20 thereof.
Dielectric strip 14 of strip electrode 10 has front and back dielectric layers 22 and 24, respectively. Each front layer 22 has a front layer opening 26 for each contact disk 16. Openings 26 are circular and somewhat smaller in diameter than contact disks 16. Front and back layers 22 and 24 are sealed together by adhesive and/or heat such that they form, in essence, an integral dielectric strip.
As can be seen in FIGS. 2 and 3, the subdural strip electrodes of the prior art are predominately rectangular in cross-section. Other subdural strip electrodes of the prior art have a circular or round cross section.
As can be seen in FIGS. 4 and 5, prior art subdural strip electrodes that have a rectangular (FIG. 4) or round (FIG. 5) cross-section do not optimize the amount of surface area of the electrode E in contact with the cortical surface S. As the electrode E is inserted between the dura D and the cortical surface S, downward pressure is exerted by the dura on the electrode, which in turn exerts pressure on the cortical surface. Such pressure causes the cortical surface S to slightly deflect downward, as shown in the Figures. Prior art rectangular or round electrodes cannot follow this deflection, resulting, in the case of a rectangular electrode (FIG. 4), in the electrode contacting the surface S primarily at the edges; and in the case of the round electrode (FIG. 5), in the electrode contacting the surface S along an arc.
A variety of other electrodes have been used for insertion into other tissue, such as the spinal cord, musculature, and heart. However, none of these electrodes have been custom-built to conform to the shape of a portion of the tissue in which they are being inserted, and thus electrical contact between the tissue and the electrode has not been optimized, because of the same type of problem as noted above with reference to subdural strip electrodes.
There is a need for a tissue-conformable electrode which can be custom-built to conform to the contours of a portion of the tissue in which or on which the electrode is being placed.